Abstract

Nominal samples with , 8, and 10 crystallized in the type-VIII clathratestructure (, No. 217), while the sample with crystallized in the type-I clathratestructure (, No. 223). While a large number of the type-I thermoelectric clathrates exist, only three type-VIII clathrates of , , and had been synthesized before. The type-VIII samples ( and ) with various carrier concentrations were prepared to investigate their thermoelectric properties. They exhibited the temperature dependences of electrical conductivities and the Seebeck coefficients typical of type degenerate semiconductors, which almost depended on their carrier concentrations systematically. A relatively large dimensionless figure-of-merit of 0.56 at 800 K was obtained for the type-VIII sample with a carrier concentration of . This value is comparable to that of 0.62 at 800 K for the type-I clathrate. The type-VIII clathrate had a smaller effective mass, a higher mobility, and a higher lattice thermal conductivity than those of the type-I clathrate. The difference in transport properties between the type-I and type-VIII clathrates is also discussed.

Received 19 December 2008Accepted 16 February 2009Published online 01 April 2009

Acknowledgments:

The authors would like to acknowledge the PC cluster resources of the Media and Information Technology Center, Yamaguchi University for calculation of the electronic structure. This work is partially supported by a Grant-in-Aid for Scientific Research (C) (Grant No. 20510106) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.